The present invention relates to an optical device for the recording or inscribing and reading of data media and to an optical memory system incorporating such a device.
The known recorded and optically readable data media in the form of disks or tapes are such that the recorded or stored data are translated by local changes of one parameter of the recording layer, e.g. its height, optical refractive index, absorption, reflection or transmission coefficient, said changes being optically detectable by means of electro-optical sensors.
Generally, the data are recorded along a track which, as a function of the configuration of the medium, is either linear (tape) or in the form of a spiral or concentric ring (disk). The invention more particularly relates to digital data storage systems incorporating disks, because only they give rapid access to a particular region where data which it is wished to find are recorded.
A well known process for recording data consists of forming very small depressions on the disk surface, which are of variable length in the direction of the tracks. This variable length representing a time modulation of the data to be recorded.
As is known, reading can take place in accordance with two basic approaches, i.e. by reflection of light rays focused on a reflecting surface (e.g. a metal deposit made on the side of the disk carrying the minute depressions) or by transmission. In both cases, the focused rays are spatially modulated by the minute depressions representing the data recorded on the red side of the disk. Photoelectric cells detect the reflected or transmitted signals. The electrical signals detected by the photoelectric cells are then processed by suitable electrical circuits and are shaped so as to restore the recorded or stored data.
The reading and also writing light rays are produced, e.g. by a laser source. The rays are focused by means of a recording-reading head incorporating focusing optics and kept at a suitable distance from the surface of the disk to be read by means of control members.
The known optically readable disks make it possible to record approximately 10.sup.10 data bits in the case of a disk with a diameter of approximately 30 cm. Such disks can be used in digital data processing means in mass memory systems. It is obvious that the aforementioned capacity is completely inadequate for this. The same difficulties are encountered when using other types of disks, particularly magnetic disks. In addition, numerous processes have been proposed for increasing the individual capacity of a disk.
Firstly, it is known to record the data on the two sides of a disk, whilst maintaining constant all the other parameters (recording density, etc).
It is also possible to act on the geometrical parameters of the disk and in particular increase its diameter. However, this diameter increase leads to disadvantages such as an increase in the system inertia, risks of offcentring causing vibrations, increase in the sag of the disk and in the dimensions of the reader-inscriber in which the disk is placed. In addition, the actual disk is more difficult to manufacture.
The increase in the disk diameter, although permitting a correlative increase in the disk capacity, is an important reason for the increase in the average access time to an area of the disk in which data are or should be stored. Complementary processes permitting a greater storage capacity have been proposed, non-limitative examples of these being recording at a constant linear speed (asynchronous disks) and at constant angular speed (synchronous disks) or prior electronic compression of the data to be recorded. However, there is a recording density limit linked with the technology used and in particular the materials, the quality of the controls for the reading means and the appearance of parasitic phenomena. The upper recording density limit can be gathered from diffraction laws in the case of optical recording. Moreover, the data compression methods can only be used at the periphery of data processing systems, e.g. for the acquisition of digitized analog data, such as data acquisition in connection with seismic prospecting.
Another known method for magnetically recording digital data is to use a plurality of disks, arranged in stacks and read simultaneously or sequentially by a plurality of magnetic heads.
This method is advantageous in connection with magnetic recording and reading because the magnetic heads have limited dimensions and weight. However, this is not the case with optical recording and reading heads which, besides the actual optical elements incorporate a position control device, at least in a direction orthogonal to the plane of the disk. This device incorporates metal pole pieces having a by no means negligible weight and size. Thus, serious difficulties are encountered when using this method in connection with optical recording so that the resulting recording apparatus is heavy and cumbersome as a result of the increase in the number of heads. The apparatus is also complex due to the present electrical connections carrying the control signals of the position control device.